1. Field of the Invention
The present invention relates to the transmission of data, in particular when the data are gathered in frames or blocks comprising headers.
2. Discussion of the Related Art
FIG. 1 shows the structure of a frame header 1 in a data transmission such as currently provided by standard DVBS2 (Digital Video Broadcasting Satellite).
Header 1 comprises 90 symbols and is formed of a portion 2 noted “SOF” (“Start Of Frame”) and a portion 3 noted “PLS” (“Physical Layer Structure”).
Portion SOF comprises 26 symbols corresponding to a known 26-bit sequence. To obtain these 26 symbols, the 26 bits of the known sequence are coded according to a modulation of π/2.BPSK type. According to this modulation, a symbol ejφ or −ejφ is assigned to each bit 0 or 1 and, further, the even and odd symbols exhibit a π/2 phase shift with respect to one another.
Portion PLS comprises 64 symbols, and defines the structure of the frame elements, like the type of data coding, the used modulation, the frame length, the presence of pilots, the pilots corresponding to a set of symbols known by the demodulator and easing the estimating of the reception parameters, etc. The 64 symbols of portion PLS originate from a 7-bit code word b0-b6 corresponding to 27, that is, 128 coding possibilities for the frame. Some bits of the code word play a particular role. Thus, bit b1 indicates whether the frame is long or short and bit b0 indicates the presence or the absence of pilots.
FIGS. 2A and 2B illustrate a way of obtaining the 64 symbols from the 7 bits of the code word. In FIG. 2A, 64 bits B0, B1, . . . Bi, . . . B63 are obtained by multiplying a line vector (b0, b6, b5, b4, b3, b2, b1) formed of the bits of the code word by a matrix M comprising 7 lines and 64 columns. Matrix M is shown in FIG. 2B. The 64 bits Bi resulting from the multiplication are then combined (by XOR) with a known word UW (“Unique Word”) to improve the spectrum distribution of the signal power and transformed into symbols by a modulation of π/2.BPSK type. The 64 obtained symbols are unknown by the receiver, but it is known on reception that they belong to a determined set of 128 possibilities.
A difficulty to decode the signal is to determine the frame headers.
For example, on a first acquisition (powering-on, television channel switching, information drop-out, as when passing under a tunnel in mobile reception), the header position is unknown and the header may start at any symbol. Account must also be taken in this case of the carrier frequency shift. This shift, due to the various frequency switchings undergone by the signal, is then unknown and its effect is to assign to each symbol a phase error proportional to time. The carrier frequency shift may be very large, to for example reach 20% of the symbol frequency.
In steady state, where the decoding of the code word of the previous header provides the frame length and accordingly the position of the next header, the code word may vary from one frame to the other and specific care must be given to the header determination.